Oxide coated cathode



April 15, 1941. J. w, MCNALL 2,238,595

OXIDE COATED CATHODE Filed Sept. 29, 1938 INVENTOR J'. IV. /Vr/V/7ZZ BWM Imm.

ATORNEY Patented Apr. 15, 1,941

OXIDE COATED CATHODE .lohn William McNall, East Orange, N. J., assgnor to Westinghouse Electric `t Manufacturing Company, East Pittsburgh, Pa., a. corporation of Pennsylvania Application September 29, 1938, `Serial No. 232,312

3 Claims.

The invention relates to cathodes and especially to oxide coated cathodes.

An object'fof the invention is to provide an oxide -coated cathode in which the emissive coating will not flake off from the core construction.

Other objects and .advantages of the invention will be apparent from the following description `and drawing, in which:

The' figure is a cross-section through an oxide` coated cathode illustrating a preferred embodi- Vment of the invention.

There has been considerable trouble encountered with the ilaking olf of the emissive coating in oxide coated cathodes, especially where such oxide coated cathodes are utilized in mercury vapor tubes. 'The emissive coating for such cathodes is usually a combination of two or more alkaline earth metals such as barium, strontium, and calcium, generally put on as a carbonate coating of the three metals, referred -to as the triple carbonate coating, which is activated into the oxides. The coating sometimes curls up like dried mud and flakes off leaving a poorer and smaller emissive area. The current concentrates on the coating adjacent to these aked off places and there results considerable sparking at these places which rapidly destroys the `coating and the emission of the cathode. Sparking also occurs at portions of the coating having poor contact with the core metal. lWhile I do not intend to be limited to the following explanation, it is my belief that this undesirable phenomenon is often caused by a very .thin layer of an oxide or compound of one of the metals inherent to the core metal which layer is formed on the core metal before 5 being coated, during activation or seasoning.

The triple carbonate coating does not adhere well to this metallic oxide or compound layer and the flaking off of the coating occurs as a result.

It is possible that the transfer of the electrons means whereby yan undesired metallic oxide or :f

compound layer will not occur between the electron emissive coating and the core metal preventing good adhesion'between the two, but on the contrary, to provide a layer which will be securely attached to the core met-al and at the (Cl. o-27.5)

msame time provide good adhesion to the electron emissive coating. In particular, I have found that `barium hydroxide will act as the priming coat which adheres well to the lcore metal and also adheres to the electron emissive coating.

Nickel `or other metals can be used as core metal, but I `preferably use a nickel cobalt titanium alloy such as described in Patent #2,030,112 to Erwin F. Lowry for Alloy and comprising to 35% nickel, 40 to` 10% cobalt, 20 to 5% ferrotitanium, because of its greater rigidity at operating temperatures. The alloy I found especially suitable was substantially 73% nickel, 17.5% cobalt, 6.5% iron, 2.5% titanium, .2% manganese, marketed under the trade name of KonaL I preferably treat the -core prior to forming the coating thereon. While the core may be hydrogen treated, I prefer instead to heat-treat the core in a Vacuum. rI'he heat treatment may consist of passing a current through the core, or in utilizing a furnace. I have found that maintaining the core in a vacuum furnace at approximately 900 for several hours, removes any undesired oxides and impurities therefrom. In order to suitably apply the barium hydroxide to the core, I utilize a vehicle for the barium hydroxide. While barium oxalate or barium nitrate, or nitrocellulose binder or other liquid might be used, I prefer to utilize an aqueous solution of barium nitrate. This solution was made by adding twenty grams of barium hydroxide to sixty -cubic centimeters of a 5% aqueous solution of barium nitrate and thoroughly mixing the two.

The preferred mixing was that of utilizing balls revolving through the solution to finely grind the barium hydroxide throughout the `aqueous solution of barium nitrate. This type of mixing is referred 4to as Ball Milling and was continued for about a total of hours.

The vacuum heat-treated core was then dipped into Athe ball-milled mixture and Was again heated in a vacuum, but this time was heated by current passing through the core metal although vacuum baking could be used. The temperature was gradually raised to 1200" centigrade and the pressure inside of the vacuum oven was less than one tenth micron of mercury. This heat treating in the vacuum resulted in the evaporation of the carrier and probably the Water of crystallization of the barium hydroxide, so that a thin coating of anhydrous barium hydroxide remained on the core metal and adhered so strongly thereto that it was impossible to even scratch the barium hydroxide o the core Without scratching off Some of the core metal with it.

The barium hydroxide coated core was then coated with a mixture of barium, strontium, and calcium carbonates which was then activated in the usual manner. In general, this consists in placing the cathode in its position inside of the tube and then baking the tube in an oven Yto as high a temperature las the glass composition will permit, generally, for soft glass tubes, around 400 centigrade. The metal parts of the anode are heated by high frequency induction and then the cathode is heated very slowly by passing current through it. During all this time, the tube is being pumped. The heating process drives 01T the carbon dioxide 'and moisture from the triple carbonate coating andleaves a barium, strontium, and calcium oxide mixture which adheres very firmly to the barium hydroxide layer on the core.

It was observed that the activation of the cathode with my barium hydroxide layer thereon was quicker and easier than it had been for the regular cathodes Without the barium hydroxide layer. The coating showed no signs of cracking or iialling and passed larger currents Without sparking than was possible with tubes not having the barium hydroxide layer on the core.

It is apparent that modifications may be made in the combination of materials forming the preferred embodiment above and illustrated in the drawing, and also that the various meth-od steps @o described might be modiiied in various particulars without departing from the spirit and scope of the appended claims.

I claim:

1. A cathode comprising a metallic conductive core, an intermediate layer of anhydrous barium hydroxide on said core and a surface layer of electron emissive material on said barium hydroxide.

2. The method of coating an area on a metal core with electron emitting material which comprises covering the `area with hydrated barium hydroxide, evaporating the Water of crystallization from said barium hydroxide to leave a layer of anhydrous barium hydroxide bound to said core and coating said anhydrous barium hydroxide layer with electron emissive material.

3. The method of firmly coating an area on a met-al core with electron emitting material containing barium an-d strontium which comprises covering the area on the metal core with a layer of finely ground barium hydroxide in a carrier, heating said layer in a vacuum to evaporate said carrier and to firmly bind said barium hydroxide layer to the core, then coating said layer with an electron emissive coating containing barium and strontium and activating said electron emissive coating.

JOHN WILLIAM MCNALL. 

